410 



Mr. R. Shida. 



wire (pianoforte wire) ; and while held horizontally by means of pliers 

 over a tray containing cold water, it was raised to a bright red heat 

 by passing throngh it a strong current from a Faure battery, and 

 suddenly plunged into the tray. This plan proved a complete success, 

 the heat being equally distributed throughout the whole mass of the 

 wire ; the tempering was, of course, as uniform as it could be all over 

 the length of the wire, perhaps, with the exception of the ends where 

 it was held. When short pieces were cut off from the extremities, 

 the wire was 78'42 centims. long ; the area of its cross-section was now 

 '004326 square centim., the wire having lost nearly 2 per cent, of its 

 weight by the process of tempering. This wire was, of course, so exceed- 

 ingly brittle that the operation of " Ons and Offs " of a heavy weight 

 was an impossibility, and consequently no weight was put on the wire at 

 all, except those used to keep it vertically straight. With reference to 

 the explanation of the Table IY, the first column in 1, 2, 3, &c, refers 

 to the result arrived at when the magnetising force was kept acting 

 on the wire ; and the second column, if there be one, refers to the 

 result arrived at directly after the withdrawal of all magnetising force, 

 except that due to the vertical component of the earth magnetism. 



Somewhat thick bars of cast iron, hard-tempered steel, and soft 

 iron, were then procured and experimented upon, with a view to 

 determine approximately the law of magnetisation of those bars, and 

 to compare the results with each other and with those for the wires. 

 The bars were nearly equal in their dimensions ; they were all 61 

 centims. in length and very nearly square in section ; the sectional 

 area of the cast-iron bar, when calculated from its weight and specific 

 gravity, was approximately *950 square centim., that of the steel 

 bar *948 square centim., and that of the soft iron bar '901 square 

 centim. 



With regard to the mode of experimenting in the case of these 

 bars, though it remained the same in principle as before, it necessarily 

 differed in details, which I proceed to describe thus : — In the first 

 place, the coil employed for magnetising the bars was 68 centims. 

 long, and consisted of three layers of insulated copper wire wo and on 

 a tube of copper nearly square, each layer containing 620 turns ; the 

 whole area inclosed by all the turns of wire per unit length was 

 89 sq. centims. approximately, though not very accurately on account 

 of the difficulty of measuring exactly the dimensions of the coil, as it 

 was not specially made for the purpose ; and the resistance of the coil 

 was about 3" 78 ohms when cool. The bar to be experimented on was 

 placed inside the coil, with its centre and axis coincident with those 

 of the latter; and the whole arrangement thus fitted up was hung 

 vertically in the same way as before by means of cords, pulleys, &c, 

 with the common axis of the coil and the bar at a distance of 22 cen- 

 tims. from, and due magnetic east of, the magnetometer ; the con- 



